Alloys which possess shape memory are well known. Articles made of such materials can be deformed from an original undeformed configuration to a second deformed configuration. Such articles revert to the undeformed configuration under specified conditions. They are said to have shape memory. One set of conditions which will enable a deformed configuration of an article having shape memory to recover towards its undeformed configuration or shape is the application of heat alone. In such an instance the material is spoken of as having an original heat-stable configuration and a second, heat-unstable configuration. The alloy is formed into the heat-unstable configuration at a temperature where it is in a predominantly martensitic phase. Upon application of heat, the article made of such a material can be caused to revert or to attempt to recover from its heat-unstable configuration towards its original heat-stable configuration, i.e., it "remembers" its original shape.
Among metallic alloys the ability to possess shape memory is the result of the fact that the alloy undergoes a reversible transformation from a predominantly austenitic state to a predominantly martensitic state with a decrease in temperature. This transformation is sometimes referred to as a thermoelastic martensitic transformation. An article made from such an alloy is easily deformed from its original configuration to a new configuration when cooled below the temperature at which the alloy is transformed from a predominantly austenitic state to a predominantly martensitic state. The temperature at which this transformation begins is usually referred to as MS and the temperature at which it finishes is usually referred to as M.sub.f. When an article thus deformed is warmed to the temperature at which the alloy starts to revert back to its austenitic state, referred to as AS (A.sub.f being the temperature at which the reversion is complete), the deformed object will begin to return to its original configuration. Many shape memory alloys (SMAs) are known to display stress-induced martensite (SIM). When a SMA sample exhibiting such SIM is stressed at a temperature above M.sub.s (so that the austenitic state is initially stable), but below M.sub.d (the maximum temperature at which martensite formation can occur even under stress), it first deforms elastically and then, at a critical stress, begins to transform by the formation of SIM. Depending on whether the temperature is above or below A.sub.s, the behavior when the deforming stress is released differs. If the temperature is below A.sub.s, the SIM is stable; but if the temperature is above A.sub.s, the martensite is unstable and transforms back to austenite, with the sample returning (or attempting to return) to its original shape. The effect is seen in almost all alloys which exhibit a thermoelastic martensitic transformation, along with the thermal shape memory effect. However, the extent of the temperature range for which SIM is seen, and the stress and strain ranges for the effect vary greatly with the alloy.
Various proposals have been made to employ shape memory alloys in the medical field. For example, U.S. Pat. No. 3,620,212 to Fannon et al. proposed the use of a SMA intrauterine contraceptive device; U.S. Pat. No. 3,786,806 to Johnson et al. proposes the use of a SMA bone plate; U.S. Pat. No. 3,890,977 to Wilson proposes the use of a SMA element to bend a catheter or cannula; U.S. Pat. No. 4,485,816 to Krumme, discloses use of a shape-memory surgical staple for use in holding the edges of a wound together while it heals; U.S. Pat. No. 4,170,990 to Baumgart shows the use of SMA's for medical purposes such as wires to correct scoliosis, disk clamps and intermedullary rods. Also, U.S. Pat. No. 4,665,906 to Jervis discloses medical devices which make use of the pseudoelastic (SIM) properties of such shape memory alloys.
These medical SMA devices rely on the property of shape memory, thermal or stress relieved, to achieve the desired effects. That is to say, they rely on the fact that when a SMA element is cooled to its martensitic state and is subsequently deformed, or when stress is applied which is causing a portion of the SMA to be in its martensitic form, it will retain its new shape; but when it is warmed to its austenitic state or the stress is relieved, as the case may be, the element recovers towards its original shape.
There are currently three different techniques used for holding the edges of a wound together while it heals. The oldest and most widely used method relies on the use of flexible threads which are passed through the apposing tissue edges and then tied. Such threads can be made of silk or of a synthetic polymeric material. Some such threads must be removed by the physician after healing has occurred while others will gradually be absorbed by the body. Another method relies on metal staples that are placed across the apposing tissue edges and then bent further inwardly to hold the wound edges together and prevent the staple from falling out. The final method relies on bands or strips which are placed across the wound with the bands or strips being held to the surface of the skin by an adhesive backing, by staples, or by sutures.
The metal staples and the bands or strips generally provide only surface closing of the wound and do not adequately hold the more deeply buried portions of the edge of the wound together.
Only sutures provide the important capability of holding together the more deeply buried portions of the edges of a wound. Such a capability is quite important since if the deep portion of a wound is not held together body fluids can accumulate and infection can occur in a region where the body's defenses cannot respond. And, the flexible sutures currently used, because of the fact that they are tied in place, provide a shear force which acts from deep in the wound to the surface of the wound as well as a force which tends to force the edges of the wound together. This shear force is not desirable in that it may interfere with proper alignment of the wound edges since it can cause the edges of the wound to pucker or bow apart. As a result, it is necessary to use two or more layers of sutures, one layer buried below the other, to adequately close a deep wound. Also, the skill of the individual surgeon determines how well currently flexible sutures will hold a wound together. And, it is difficult and time consuming to properly tie each of a series of successive sutures in a wound so as to provide the best possible closure of the wound.
The present invention is directed to overcoming one or more of the problems as set forth above.